As disclosed in JP, A, 1-275902, there is conventionally known a hydraulic drive system for construction machines in which a pressure compensating valve provided in a center bypass line of a valve group gives a load compensating function to directional control valves included in the valve group. This prior hydraulic drive system comprises a hydraulic pump of variable displacement type, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, a valve group including a plurality of directional control valves of center bypass type for controlling respective flows of the hydraulic fluid supplied from the hydraulic pump to the plural hydraulic actuators, center bypass line for connecting in series center bypasses of the plural directional control valves to a reservoir, a plurality of bleeding-off variable restrictors respectively disposed in the center bypasses of the plural directional control valves to reduce their opening areas as input amounts of the corresponding directional control valves increase, a pressure compensating valve provided in the center bypass line at a position downstream of the valve group, first and second differential pressure detecting lines connected to the center bypass line for transmitting a differential pressure to the pressure compensating valve, a fixed restrictor provided in the center bypass line at a position downstream of the pressure compensating valve for producing a control pressure, and a pump regulator for changing the displacement volume of the hydraulic pump dependent upon the control pressure.
One of the first and second differential pressure detecting lines is connected to the center bypass line at a position upstream of the valve group, while the other line is connected to the center bypass line at a position downstream of the valve group.
In the hydraulic drive system thus arranged, the pump regulator for controlling the displacement volume of the hydraulic pump performs well-known negative control dependent upon the control pressure produced by the fixed restrictor. More specifically, as the amount of stroke of the directional control valve increases, the opening area of the bleeding-off variable restrictor is gradually reduced and fully closed at last. During this process, the flow rate of the hydraulic fluid passing through the center bypass line is reduced to make smaller the control pressure produced by the fixed restrictor and, correspondingly, the pump regulator is operated to gradually increase a delivery rate of the hydraulic pump. A metering characteristic of the hydraulic fluid supplied to the actuator is determined by both the pump flow rate characteristic and the characteristic of the bleeding-off variable restrictor in the above process.
Stated otherwise, when one of the plural directional control valves is operated, the delivery rate of the hydraulic pump is increased with the spool stroke increasing, as mentioned above. At the same time, as the spool stroke increases, the larger will be the opening areas of a meter-in variable restrictor and a meter-out variable restrictor of the directional control valve and the smaller will be the opening area of the bleeding-off variable restrictor. Therefore, the flow rate of the hydraulic fluid flowing from the hydraulic pump out to the reservoir through the center bypass line is reduced to make higher a delivery pressure of the hydraulic pump. Then, at the time the pressure at a pump port of the directional control valve becomes higher than the load pressure imposed on the actuator, the hydraulic fluid from the hydraulic pump begins to flow into the actuator side and, thereafter, the flow rate of the hydraulic fluid flowing from the pump out to the reservoir through the center bypass line is further reduced. Correspondingly, the flow rate of the hydraulic fluid flowing into the actuator side, i.e., the flow rate resulted by subtracting, from the pump flow rate, the flow rate of the hydraulic fluid flowing out to the reservoir through the center bypass line, is increased. This is generally called bleed-off control.
In addition, the pressure compensating valve provided in the center bypass line makes control so that a differential pressure across the bleeding-off variable restrictor of each directional control valve is held constant. Therefore, the flow rate of the hydraulic fluid flowing out to the reservoir through the bleeding-off variable restrictor is determined in magnitude by the opening area of the bleeding-off variable restrictor (i.e., the amount of stroke of the directional control valve) regardless of the magnitude of the pump delivery pressure, that is to say, the magnitude of the load pressure. Consequently, the flow rate of the hydraulic fluid flowing into the actuator side will not be affected by the load pressure, thus providing the so-called load compensating characteristic.